Salt damage of plants is a phenomenon in which the salts in soil for cultivating plants such as crops accumulate at the earth surface, rendering crop growth impossible. Currently, it is said that 10% of irrigated agricultural land in the world. i.e. approximately 210,000 km.sup.2 of irrigated agricultural land, is affected by salt damage. For example, irrigated agriculture is generally conducted in the dry areas of various countries in the world, such as Pakistan, Southern parts of the United States, Egypt, Saudi Arabia and China.
When conducting irrigated agriculture in such dry areas, irrigated water penetrates into the earth and salts dissolve in the earth. The abundant sunlight, characteristic of dry areas, evaporates a lot of water from the earth surface, and underground water containing salts rises to the earth surface via the capillary phenomenon. The salts which have risen to the earth surface do not evaporate but accumulate at the earth surface, impede crop growth and eventually kill the crops. In particular, vegetables, melons, peppers and tomatoes suffer from serious salt damage.
Such salt damage has occurred since ancient times. For example, it is said that the cause of the fall of the ancient Mesopotamia civilization may have been salt damage. While the capillary phenomenon of salt-containing water can be regulated to a certain extent in areas where continuous irrigation is possible, salt damage is very serious in areas where only intermittent irrigation is possible. In areas where the temperature is very high and areas where rock salt lies underground, situations eventually reach the point at which agriculture must be abandoned.
There have been methods proposed to prevent this salt damage, such as a method which washes the soil with a large quantity of irrigation water, a method which replaces the soil with new soil, and a method which controls water evaporation by laying straw and such on the earth's surface. However, these methods not only require a lot of work and expense but also they are not methods which fundamentally prevent salt damage. Once salts accumulate at the earth surface, restoration of the land requires a lot of work, and thus crop production is doomed.
Recently, for the purpose of harvesting vegetables and fruits throughout the year, facility cultivation, particularly greenhouse cultivation using vinyl greenhouses, has become popular.
In facilities such as vinyl and glass greenhouses, the ceilings are covered with vinyl or glass to keep the room temperature and the soil temperature at appropriate levels even in winter, and the temperature is maintained by burning kerosene and such in winter. Also, since natural rainfall is blocked, periodic sprinkling of water is conducted to allow the harvesting of vegetables and fruits throughout the year.
As the greenhouse culture has spread throughout Japan as described above, there has arisen the problem of the accumulation of high concentration salts in the soil in greenhouses (greenhouse soil) which have affected the harvests.
That is, artificial water sprinkling is conducted in greenhouses because the ceilings prevent natural rains from wetting the soil. However, the amount of the water sprinkling is limited to the minimum required level for the crops, and thus the artificial water sprinkling is always kept rather low. Because of this, the wash-purge action of soil by rainfalls, such as for the outdoor soil, is insufficient, and salts in lower soil layers tend to be carried with water ascending through the soil because of the capillary phenomenon, resulting in accumulation at the soil surface. When intermittent sprinkling is conducted rather than drip sprinkling, water moves up and down in the soil to aggravate salt and metal ion accumulation.
Furthermore, to harvest more crops, greenhouse soil is always supplied with a rather large quantity of chemical fertilizers. Decomposed chemical fertilizers release sodium ions, calcium ions, nitrate ions, etc. and these ions, which would be naturally washed-purged out into lower layers in outdoor natural soil, however in greenhouse soil, accumulate at the soil surface because of the capillary phenomenon, resulting in extremely salty soil.
When such salty soil occurs, it not only causes excess-salt disorders, but causes abnormalities in the soil environment surrounding the roots, such as the degradation of the microorganism fauna, and plant diseases due to abnormally high absorption of metal ion, which is one of the causes of crop production instability.
Conventionally, improvement measures against such salt accumulation, water-pouring treatments, cultivation of cleaning crops, application of coarse organic materials, desalting by removal of the covering, soil removal, soil import, etc. have been carried out. However, since these methods require a lot of work and expense, new methods have been sought.